Procedure

1. Copy the sendmail.mc file and rename it sendmail.d.mc for later use.

   # cp -p /etc/mail/sendmail.mc /etc/mail/sendmail.d.mc

2. In the sendmail.mc file, add the following statements before MAILER(smtp)dnl to relay all email messages to IMSS:

   define(`SMTP_MAILER_ARGS',`TCP [127.0.0.1] 10025')dnl
   MODIFY_MAILER_FLAGS(`SMTP', `+k') dnl
   define(`ESMTP_MAILER_ARGS',`TCP [127.0.0.1] 10025')dnl
   MODIFY_MAILER_FLAGS(`ESMTP', `+k') dnl
   define(`SMTP8_MAILER_ARGS',`TCP [127.0.0.1] 10025')dnl
   MODIFY_MAILER_FLAGS(`SMTP8', `+k') dnl
   define(`RELAY_MAILER_ARGS',`TCP [127.0.0.1] 10025')dnl
   MODIFY_MAILER_FLAGS(`RELAY', `+k') dnl
   MODIFY_MAILER_FLAGS(`LOCAL', `+k') dnl
   define(`LOCAL_MAILER_PATH',`[IPC]')dnl
   define(`LOCAL_MAILER_ARGS',`TCP [127.0.0.1] 10025')dnl
   

3. Change the existing DAEMON_OPTIONS setting in the sendmail.mc file into the following to receive SMTP requests from all hosts:
   DAEMON_OPTIONS(`Port=smtp, Addr=0.0.0.0, Name=MTA')dnl

   If necessary, add the following DAEMON_OPTIONS setting to enable IPv6 support:

   DAEMON_OPTIONS(`Port=smtp, Addr=<IPv6_address>, Name=MTA_IPv6, Family=inet6')dnl
4. Run the following command for the sendmail.cf file to take effect:

   # m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf